1. Preheat oven to 170°C. Grease deep 22cm round cake pan. Stir butter, dark chocolate and the water in medium saucepan over low heat until smooth. Transfer mixture to large bowl, cool 10 minutes.

2. Meanwhile beat eggs and sugar in a small bowl with an electric mixer until thick and creamy, stir into chocolate mixture. Stir in sifted dry ingredients, pour batter into prepared pan.

3. Bake uncovered, about 1¼ hours. Stand cake in pan 10 minutes. Turn cake onto wire rack to cool. Enclose cake with plastic wrap, refrigerate 3 hours or overnight.

4. Split cake horizontally, third of the way from the top. Place bottom two thirds of cake on serving plate, using small teaspoon or melon baller, scoop about 14 deep holes (at equal distances apart and not through to bottom of cake) out of cake, reserving 1¼ cups of scooped-out cake pieces for truffles.

Prepare dark chocolate ganache. Heat the cream in a saucepan until just boiling. Pour the cream over dark chocolate, stir until smooth. Leave dark chocolate ganache to cool until it reaches the desired consistency.

Prepare the white chocolate ganache. Heat the cream in a small saucepan until just boiling. Pour the cream over the white chocolate and leave to stand for 3 minutes. This will give the white chocolate time to soften. Stir the ganache until smooth and cool until it reaches the desired consistency.

1. For the white chocolate custard, place milk in small saucepan. Split vanilla bean in half lengthways, scrape seeds into saucepan, then add pod, bring to a boil. Cool 10 minutes, discard pod.

2. Whisk in egg yolks, transfer mixture to medium heatproof bowl and bain-marie for 10 minutes or until it coats the back of a spoon.

3. Sprinkle gelatin over the water in small heatproof jug; stand jug in same saucepan of simmering water. Stir until gelatin dissolves then pour into milk mixture, cover, cool 5 mins.

4. Add chocolate and cream, stir until smooth. Pour immediately into holes in cake. Cover with dark chocolate ganache, then replace top of cake, cover, refrigerate 2 hours.

1. For truffles, stir chocolate and cream in small saucepan over low heat.

2. Stir in reserved scooped-out cake, cover, refrigerate until firm.

3. Roll level teaspoons of the mixture into balls, place on trays, cover. Refrigerate until firm.

4. Spread white chocolate ganache over cake, decorate with truffles.